NEAR FIELD OPTICAL RECORDING DEVICE AND A METHOD OF OPERATING A NEAR FIELD OPTICAL RECORDING DEVICE
A near field optical recording device and method of operating a near field optical recording device, the device being arranged to cooperate with an optical record carrier. The device comprising means to adjust tilt of a refractive optical element with respect to the optical record carrier according to a distance between the refractive optical element and the data layer of the optical record carrier to be accessed.
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The invention relates to the field of near field optical recording devices. In particular, the invention relates to a near field optical recording device comprising:
a light source,
a refractive optical element arranged to direct light from the light source towards the optical record carrier,
a tilt error servo loop comprising a tilt error signal related to the tilt of the refractive optical element with respect to the optical record carrier,
and a tilt servo gain setting, applied to the tilt error signal.
BACKGROUND OF THE INVENTIONThe maximum data density that can be recorded on an optical record carrier in an optical recording device inversely scales with the size of the laser spot that is focused onto the optical record carrier. The spot size is determined by the ratio of two optical parameters, namely, the wavelength of the light source of the device (usually a laser) and the numerical aperture (NA) of the refractive optical element (often an objective lens) employed to direct light from the light source onto an optical record carrier. In conventional optics, the NA is limited to a value smaller than 1.0. In near field optical recording, however, the NA can be made larger than 1.0 by use of e.g. a solid immersion lens (SIL) as the refractive optical element, thus allowing further extension to larger storage densities of data on an optical record carrier. An NA greater than 1.0 is only available within an extremely short distance form the SIL, a distance typically smaller than one tenth of the wavelength of the light. As a result the SIL and optical record carrier must be kept within a few ten's of nanometers of each other during operation. The distance between the refractive optical element and the optical record carrier (the air gap) is accurately controlled by an air gap control system using conventional focus and tracking actuator in combination with a very sensitive gap error signal, which is derived from a polarization detection of the reflected light.
A near field optical recording system is further described in Optical Data Storage 2004, Proceedings of SPIE Vol 5380, pp 209 to 223, F. Zijp et al where a device with system NA 1.9 is arranged to cooperate with a 50 GB optical record carrier without cover layer (first surface optical record carrier).
More general information on optical heads can be found in the Encyclopaedia of Optical Engineering DOI: 10.1081/E-EOE 120009664 (2003), Marcel Dekker Inc. Sections on tracking actuators, servo mechanisms, objective lens and advances in light paths are of particular relevance to this application.
Various kinds of optical record carriers can be used in conjunction with a near field-recording device. Some optical record carriers comprise a cover layer to protect data stored on the optical record carrier.
A problem with known near field optical recording devices is that the refractive optical element must be aligned very accurately with respect to the optical record carrier.
SUMMARY OF THE INVENTIONIt is an object of the invention to provide tilt control of the refractive optical element in a near field optical recording device, which is capable of providing better tilt control than for conventional systems, thereby improving alignment of refractive optical element with respect to the optical record carrier.
This object is achieved according to the invention in that the tilt servo gain setting is adjustable in response to a change in a distance between the refractive optical element and the at least one data layer.
In a further embodiment of the invention, the near field recording device further comprises a means for tilt servo gain adjustment for implementing at least one pre-determined value of the tilt servo gain in response to a change in a distance between the refractive optical element and the at least one data layer.
The control of tilt of the refractive optical element which is employed to direct light onto an optical record carrier in an optical recording device has been investigated for conventional optical systems. Near field recording systems, however, have special problems associated with the tilt due to the proximity required between the refractive optical element and the optical record carrier for near field operation. Special problems also occur when using a SIL lens due to the shape. Most SIL lenses are hemispherical or super hemispherical. In order to obtain sufficiently large margins for tilt of the optical record carrier with respect to the flat side of the SIL at such small working distances, the SIL usually has a conical shape with a small flat tip (often around 40 μm in diameter). However, even when the closest surface of the SIL is reduced in size from a millimeter to a few ten's of microns, the maximum allowed tilt angle is still very small (typically 0.070 up to about 0.20). This is quite different from current optical recording devices, where much larger mechanical tilts of the optical record carrier are possible (for example up to 10 or more). For near field systems mechanical tilt margins are much smaller, so that it is difficult and expensive to accurately measure and correct disk tilt directly. Moreover, even a perfect alignment of the disk with respect to the optical axis may lead to a system failure if the front surface of the SIL is not exactly perpendicular to the optical axis (due to the manufacturing process, this indeed can be the case). In experimental near field devices, alignment can require tedious trial and error methods of alignment. This would not be acceptable in a commercial drive.
Earlier solutions to the problem of tilt, such as defocused spot methods or multi-spot methods all have the disadvantage that the alignment between SIL and disc needs to be sufficiently good before they can be used effectively. Because of the very tight mechanical tilt margins, this requires a pre-alignment step. This pre-alignment needs to be done on a static optical record carrier, and it is necessary to bring the lens and the carrier into contact to do the pre-alignment measurements (tilt measurement for pre-alignment can be done by using one of the earlier inventions, or by simply minimizing the GES signal level). Pre-alignment on a static disc is a disadvantage, as it increases the start-up time of the drive. Moreover, bringing the lens and disc into contact can potentially lead to damage and failure of the system.
In the invention use is made of the tilt error servo loop. This subsystem of the near field optical recording device is a means to adjust the tilt of the refractive optical element (used to direct light onto the optical record carrier) by detecting tilt and then using this as input for movement of the refractive optical element. This movement of the refractive optical element may be done using an actuator, for example. Thereby adjustment and compensation of the tilt is achieved. The conventional tilt error servo loop comprises a tilt error signal and a tilt error gain setting. The tilt error gain setting on prior art systems is factory adjusted in a commercial device to an optimum value for the device, which is independent of the distance between the refractive optical element and the optical record carrier. In the invention this gain setting is made adjustable by a means for tilt servo gain adjustment. A pre-determined value or pre-determined values for the tilt servo gain in different circumstances are made available and are implemented by the means for tilt servo adjustment. The values are determined and implemented according to the distance between the refractive optical element and a data layer of the optical record carrier. At different distances, different values of tilt gain setting are optimum for the operation of the near field optical recording device. (This will be explained later). Use of more appropriate gain setting values increases the accuracy of the tilt adjustment. As the invention can be implemented at different distances, it is then also possible to avoid a pre-alignment or coarse tilt adjustment step.
In a further embodiment of the invention, the distance between the refractive optical element and the at least one data layer comprises an air gap between the refractive optical element and a top surface of the optical record carrier. The air gap distance affects the sensitivity of the tilt signal generated to measure the tilt. The closer the refractive optical element is to the optical record carrier, the stronger the tilt signal becomes, although it remains periodic with the rotation frequency of the optical record carrier according to the fluctuations of the carrier. This behavior is seen for single light spots used for generation of tilt signals and for multispot arrangements.
The air gap in a near field optical recording system is often controlled using a gap error signal (GES). The gap servo system is started at relatively large distances of the refractive optical element from the optical record carrier, e.g. as couple of hundred nm's. At these distances the accuracy requirement for tilts is less tight than when the refractive optical element is at a working distance of e.g. 40 nm. Using the tilt signals at a particular distance a first correction of the tilt can be made. On a stationary disc the tilt signal will be d.c., on a rotating disc, the tilt signals will be a.c. in nature. Under control of the gap error signal, the refractive optical element cane be moved towards the optical record carrier. The tilt signal will change as the distance changes, and a pre-determined value of tilt servo gain can be implemented in the tilt servo loop by the means for tilt servo adjustment to optimize the tilt correction performance. One example of this optimization is adjustment of the tilt servo gain so that an overall gain setting of the tilt servo loop is adjusted to an optimum value. These steps can be repeated and incremented until the refractive optical element is at the desired working distance from the optical record carrier, with each step the tilt adjustment is tuned to tighter tolerances. The steps can also be made more frequent until the adjustment process is continuous and “on-the-fly”.
In a further embodiment of the invention, the distance between the refractive optical element and the at least one data layer comprises a layer depth between a top surface of the optical record carrier and the at least one data layer. In conventional optical devices focus of the device on a data layer is achieved by moving the objective lens of the device closer to or further from the optical record carrier. Due to the constraints of near field systems, where the near field is only available in close proximity to the optical record carrier, this mechanism for focus is not available. As the near field system moves focus from one layer to another layer on a multilayer optical record carrier (effectively a change in distance between the refractive optical element and the data layer under consideration), the size of the light spot at the exit face of the refractive optical element changes. This affects the tilt signal (see later). Provision of pre-determined values of tilt servo gain compensate for the changing spot size and allow consistent performance of the tilt compensation.
In a further embodiment of the invention, the distance between the refractive optical element and the at least one data layer comprises a layer depth of a cover layer of the optical record carrier. Optical record carriers are of different types. Some are so-called first surface discs where the data is read or written on the uppermost surface of the optical record carrier. Others comprise a coverlayer, which is a transparent layer placed over the data layer in order to protect the data from damage and dirt. The coverlayers can vary in thickness. This results in a distance variation between the refractive optical element and the data layer, which also affects the focus of the near field optical recording device on the data layer. Similar spot size changes and sensitivities in the tilt signals as described above can be influenced by provision of pre-determined values of tilt servo gain to allow consistent performance of the tilt compensation.
In a further embodiment of the invention, the at least one pre-determined value of the tilt servo gain is stored in a non-volatile memory component of the device. The values of tilt servo gain can be derived from measured or calculated sensitivity values for the tilt signal as dependent on e.g. air gap distance or coverlayer thickness. These values can be made available for use as part of the invention by storing the values in a non-volatile memory in the near field optical recording device. The value or values can then be retrieved at the appropriate moment when the distance of refractive optical element to data layer is at a point which corresponds to that related to the value or values of tilt servo gain.
In a further embodiment of the invention, the at least one pre-determined value of the tilt servo gain is automatically calculated. During a focus jump from one layer to the other, it is advantageous to simultaneously/synchronously adapt the tilt servo gain settings. For example, these intermediate settings can be derived by interpolating from the settings for the current and the next layer. In this way, the tilt servo performance, such as e.g. stability, remains optimal (i.e. same overall gain).
A method of operating a near field optical recording device, comprising the following steps:
Providing a means for tilt servo gain adjustment
Determining the distance between the refractive optical element and the at least one data layer
Using the means for tilt servo gain adjustment to apply the tilt servo gain setting in response to this distance
In a further embodiment of the invention, the method of operating an optical recording device comprises the following additional step:
Providing at least one pre-determined value of the tilt servo gain
The invention will be further elucidated with reference to the figures.
The layout of the optical path of a typical near field recording device is shown in
The tilt signal generated and detected 5 is subject to variation depending on the distance between the SIL lens 13 and the optical record carrier 8. This distance may take the form of an air gap between the SIL lens 13 and the optical record carrier 8, or it may take the form of a depth within the optical record carrier such as a cover layer thickness or a change in distance to move from one data layer to another.
The graph of
The specific example in
In order to achieve consistent and accurate tilt adjustment, the invention provides for a means for tilt servo gain adjustment which implements one or more values of tilt servo gain, thereby compensating for changes in the tilt signal caused by distance changes.
In
In
- 1. Laser
- 2. Beam shaping optics
- 3. Non polarizing beam splitter
- 4. Polarizing beam splitter
- 5. System for gap error signal and tilt detection
- 6. System for RF data and push pull signals
- 7. Forward sense detector
- 8. Optical record carrier
- 9. Quarter wavelength plate
- 10. System of lenses for focus adjustment
- 11. Arrow indicating direction of focus adjustment
- 12. System of lenses
- 13. SIL lens
- 21. Arrow indicating one period of rotation of the optical record carrier
- 31. Line indicating effect for SIL tip diameter of 40 μm
- 32. Line indicating effect for SIL tip diameter of 100 μm
- 33. Curve showing numerical example of accuracy of tilt signal with respect to air gap
- 41. Multilayer optical record carrier
- 42. Multilayer optical record carrier
- 43. Data layer
- 44. Data layer
- 45. Data layer
- 46. Data layer
- 47. Data layer
- 48. Data layer
- 49. SIL lens
- L1, L2, L3 and L4 light beam
- A1, A2, A3 and A4 arrow indicating defocused light spot size at SIL exit face
- AG air gap
- 61. Method step according to the invention
- 62. Method step according to the invention
- 63. Method step according to the invention
- 64. Method step according to the invention
- 71. Tilt servo loop
- 72. Input signals to tilt servo loop
- 73. Means for tilt servo gain adjustment
- 74. Output for tilt compensation
Claims
1. A near field optical recording device, the device being arranged to cooperate with an optical record carrier comprising at least one data layer, the device comprising: characterized in that,
- a light source,
- a refractive optical element arranged to direct light from the light source towards the optical record carrier,
- a tilt error servo loop for providing a tilt error signal related to the tilt of the refractive optical element with respect to the optical record carrier,
- and a tilt servo gain setting, applied to the tilt error signal,
- the tilt servo gain setting is adjustable in response to a change in a distance between the refractive optical element and the at least one data layer.
2. A near field optical recording device, as claimed in claim 1, the near field optical recording device further comprising a means for tilt servo gain adjustment for implementing at least one pre-determined value of the tilt servo gain in response to a change in a distance between the refractive optical element and the at least one data layer.
3. A near field optical recording device as claimed in claim 1, wherein the distance between the refractive optical element and the at least one data layer comprises an air gap between the refractive optical element and a top surface of the optical record carrier.
4. A near field optical recording device as claimed in claim 1, wherein the distance between the refractive optical element and the at least one data layer comprises a layer depth between a top surface of the optical record carrier and the at least one data layer.
5. A near field optical recording device as claimed in claim 1, wherein the distance between the refractive optical element and the at least one data layer comprises a layer depth of a cover layer of the optical record carrier.
6. A near field optical recording device as claimed in claim 1, wherein the at least one pre-determined value of the tilt servo gain is stored in a non-volatile memory component of the device.
7. A near field optical recording device as claimed in claim 1, wherein the at least one pre-determined value of the tilt servo gain is automatically calculated.
8. A method of operating a near field optical recording device, comprising the following steps:
- providing a means for tilt servo gain adjustment
- determining the distance between the refractive optical element and the at least one data layer
- using the means for tilt servo gain adjustment to apply the tilt servo gain setting in response to this distance
9. A method of operating an optical recording device according to claim 8, comprising the following additional step:
- providing at least one pre-determined value of the tilt servo gain
Type: Application
Filed: Apr 12, 2007
Publication Date: Jun 4, 2009
Applicant: KONINKLIJKE PHILIPS ELECTRONICS N.V. (Eindhoven)
Inventor: Coen Adrianus VERSCHUREN (Eindhoven)
Application Number: 12/298,149
International Classification: G11B 7/00 (20060101);